In this paper, alternative reservoir operation models under different environmental operating conditions were developed to analyze the impacts of applying different policies in a multi-reservoir system in order to balance human and environmental requirements. Three scenarios/models were developed under four sub-scenarios/operating conditions. The scenarios were: (1) an optimization model to maximize the hydropower production, (2) an optimization model to minimize the squared of the difference between the release and need, (3) a simulation model under the Hydropower Standard Operating Policy. The sub-scenarios were developed as follows: (i) no environmental flow, (ii) minimum environmental flow, (iii) environmental flow bounded by the minimum and maximum flow, and (iv) maximum environmental flow. Hydropower production and system performance criteria were calculated and compared in all cases. Moreover, the Range of Variability Approach was used to assess the hydrological alterations of each of the twelve cases. The results in a two reservoir cascade of Seimare-Karkheh, located within the Karkheh River Basin in Iran, showed that sub-scenario 3 performed best in all three scenarios. Further comparison indicated that scenario 1, under sub-scenario 3, was a good compromise solution, as it provided adequate hydropower production and performance criteria and the least hydrological alterations.